The present invention relates to a sensor element for determining the oxygen concentration in gas mixtures and also relates to a method for manufacturing such sensors.
An oxygen sensor, also referred to as a broadband lambda probe, is normally used today for controlling the air-fuel ratio of combustion mixtures for motor vehicle engines. This sensor relies on the interaction of an electrochemical pump cell and a concentration cell. With the aid of the electrodes of the pump cell, oxygen is pumped from a measuring gas chamber of the sensor into the exhaust gas stream, or from the exhaust gas stream into the measuring gas chamber. To this end, one of the pump electrodes is deposited in the measuring gas chamber, and one is deposited on the outer surface of the sensor element and exposed to the exhaust gas stream. The electrodes of the concentration cell are arranged so that one is situated in the measuring gas chamber, but the other is situated with the in a reference gas channel normally filled with air. This arrangement allows the oxygen potential of the measuring electrode in the measuring gas chamber to be compared with the reference oxygen potential of the reference electrode, in the form of a measurable voltage applied at the concentration cell. With regard to measuring technique, the pump voltage to be applied at the electrodes of the pump cell is selected so as to maintain a predetermined voltage value at the concentration cell. The pump current flowing between the electrodes of the pump cell is utilized as a test signal proportional to the oxygen concentration.
The measuring gas chamber and the reference gas channel are usually positioned in different planes of the sensor element, so that the reference gas channel is located underneath the measuring gas chamber. However, this requires at least one additional, solid electrolyte layer, which contains the reference gas channel. German Patent Application No. 196 47 144 describes, at least as a variant, an element for measuring the air-fuel ratio, where the reference gas channel is situated in the same layer plane as the measuring gas chamber. However, in the case of such a layer, experience shows that a minimum layer thickness is dependent on stamping processes during manufacture. In addition, the modified arrangement of the gas chambers creates problems relating to measuring technique, since such an arrangement increases the internal resistance of the concentration cell, and results in a one-sided loading of the measuring and reference electrodes.
The sensor element and method according to the present invention, respectively, have the advantage that the layer thickness of the layer containing both the measuring gas chamber and the reference gas channel can be varied. A layer that has a very low layer thickness, or a layer having very filagree-like boundaries of the gas chambers contained therein, and having supporting elements not connected to the boundaries may be attained.
The effect of adapting the partition situated between the measuring gas chamber and the reference gas channel on the geometry of the measuring electrode situated in the measuring gas chamber, is such that only small clearance exists between the measuring gas chamber and the reference gas channel. Therefore, the internal resistance of the sensor-element concentration cell is decreased. Furthermore, it is advatageous to design the reference electrode, located in the reference gas channel, in such a manner that it adapt to the geometry of the partion between the measuring gas amber and the reference gas channel. Also the surface of the reference electrode facing in the direction of the partition is as large as possible. This permits a uniform loading of the entire electrode surface, and decreases the electrical resistance of the concentration cell that is made of the measuring electrode and the reference electrode. This is achieved in an advantageous manner when the measuring electrode is circular and the reference electrode is led around the measuring gas chamber, which is circular as well. In addition, the internal resistance of this sensor element""s concentration cell exhibits an easily-evaluated temperature dependence, which can be used to control the temperature of the sensor element.
In another exemplary embodiment, the measuring and pump electrodes, which are usually arranged separately in the measuring gas chamber, are advantageously combined into one electrode. This allows one layer plane to be dispensed with, and further simplifies the sensor design.
By appropriately designing the layer assembly, the resistance heater intended for the sensor element into the sensor element can be incorporated so that the resistance heater is equidistant from the two large surfaces of the sensor element. This results in low mechanical stresses especially on the heater-side edges of the sensor element during the heating phase and during operation.